Fixed issues after merging latest main, as well as some edge cases I didn't notice before.

The structure of InspectorWidget::m_groups changed, so I had to update my new code accordingly.
Updated the InspectorWidget::m_groups code a bit to be more readable.
Discovered the initial property group visiblity state wasn't being set correctly when a material was first opened, so groups weren't initially hidden when they should have been. This had to be fixed in different ways for MaterialEditor's inspector and MaterialComponent's inspector.

ATOM-14688 Disable Individual Layers

Gems/Atom/Tools/MaterialEditor/Code/Source/Window/MaterialInspector/MaterialInspector.cpp

@@ -198,6 +198,11 @@ namespace MaterialEditor
                 &group, &group, group.TYPEINFO_Uuid(), this, this, GetGroupSaveStateKey(groupNameId),
                 [this](const auto source, const auto target) { return CompareInstanceNodeProperties(source, target); });
             AddGroup(groupNameId, groupDisplayName, groupDescription, propertyGroupWidget);
+            
+            bool isGroupVisible = false;
+            MaterialDocumentRequestBus::EventResult(
+                isGroupVisible, m_documentId, &MaterialDocumentRequestBus::Events::IsPropertyGroupVisible, AZ::Name{groupNameId});
+            SetGroupVisible(groupNameId, isGroupVisible);
         }
     }